Currently, image data based on various different color spaces is used in image data processing apparatuses such as computers, printers and digital still cameras. For example, in computers, image data based on the RGB color space, which presumes output to a monitor, is normally used, and in digital still cameras (DSCs), since created image data is compressed and stored in JPEG format, image data based on the YCbCr color space adapted to compression is used. Therefore, when image data created by a DSC is processed by a computer, for example, the color space of the image data needed to be converted from the YCbCr color space to the RGB color space.
However, when the color space of image data is converted from a first color space (YCbCr) to a second color space (RGB), color values arrayed at equal intervals in the first color space are not always equal intervals in the second color space, so there is the problem that tone number (reproductive color number) of converted image data is reduced compared to tone number (reproductive color number) of image data before conversion. When tone number is reduced, the number of colors that can be reproduced is reduced, so the colors of the image data cannot be reproduced correctly, creating the problem that only output images with skipped tones can be obtained.